High temperature gas generant

ABSTRACT

A propellant for a micro-gas generator, the propellant having 5-aminotetrazole, aluminum, a binder and an oxidizer and is substantially free from azodicarbonamide. This propellant substantially maintains its ballistic performance regardless of being aged at higher temperatures such as 120° C. Further, a method of manufacturing a propellant for a micro-gas generator, the method including providing a propellant mixture of 5-aminotetrazole, aluminum powder, potassium perchlorate, ethyl cellulose and fluoropolymers; adding acetone as a solvent to solvate ethyl cellulose and fluoropolymers; evaporating the acetone using a low level of vacuum until the mixture is a damp cake; and granulating and drying the damp cake forming dried propellant granules.

FIELD OF THE INVENTION

The invention relates to a propellant for use in a micro-gas generatorand a method of manufacturing such a propellant.

BACKGROUND OF THE INVENTION

In the prior art gas generants are known for various safety devicepurposes, specifically for use as safety devices applications such asseat belt retractors, seatbelt pre-tensioners, buckle pre-tensioners,air bag inflators, head rest actuators, seat interlocks, hood liftersand other pedestrian protection devices that require high reliabilitygas generation devices. For example, for seatbelt pre-tensioners, thegas produced is designed to actuate a rack and pinion style device tobetter position the vehicle occupants, prior to airbag deployment, inthe event of a crash. A known example of such propellants, as forinstance described in the U.S. Pat. No. 6,964,715, is manufactured bySpecial Devices, Inc. and known as “Green Global Gas Generant”containing 19%±1% 5-aminotetrazole, 17%±1% azodicarbonamide 1%±0.2%aluminum powder, 60%±1% potassium perchlorate, and 3%±0.5% ethylcellulose. It has been discovered that these prior art gas generants donot tolerate extended exposure to high temperatures in the range of 120°C. without changing its gas generation properties such as its ballisticperformance, including specifically how much gas is generated over acertain time period.

In particular, it was discovered that propellants like the “Green GlobalGas Generant” exhibit a change in ballistic performance after thesepropellants were exposed to high temperatures over a certain time span.Such high temperature aging resulted in altering the ballisticperformance towards a steep pressure increase over a certain time periodlike 2 milliseconds (ms) that may not be desirable since a more gradualpressure increase is desired.

Therefore, for some applications, specifically installing the micro-gasgenerator in areas where high temperatures are to be expected like forinstance in the engine compartment of a vehicle, it is desirable tocreate a micro-gas generator tolerating such high temperatures over along time period without changing its ballistic properties. A particularparameter of interest for the ballistic performance is the “quickness”,i.e. the rate at which gas is generated, or put in other words, how muchgas is generated during a certain time period such as for example over atime period of 8 milliseconds (8 ms).

SUMMARY OF THE INVENTION

According to one aspect of the invention a propellant for a micro-gasgenerator is provided, the propellant comprising 5-aminotetrazole,aluminum, a binder and an oxidizer and is substantially free fromazodicarbonamide.

According to another aspect of the invention, a method for manufacturinga propellant for a micro-gas generator is provided, comprising:providing a propellant mixture of 5-aminotetrazole, aluminum powder,potassium perchlorate, ethyl cellulose and fluoropolymers; addingacetone as a solvent to solvate ethyl cellulose and fluoropolymers;evaporating the acetone using a low level of vacuum until the mixture isa damp cake; and granulating and drying the damp cake forming driedpropellant granules.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment, the oxidizer comprises potassiumperchlorate.

According to another preferred embodiment, the binder comprises ethylcellulose as a binder constituent.

According to another preferred embodiment, the binder comprisesfluoropolymers as a binder constituent.

According to a preferred embodiment, the fluoropolymers of the binderare terpolymers of at least one of a group consisting of copolymers ofhexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE).

According to another preferred embodiment, the propellant comprises30-34% 5-aminotetrazole.

According to another preferred embodiment, the propellant comprises0.6-1.4% aluminum powder.

According to another preferred embodiment, the binder comprises 0.6-1.4%terpolymers of the at least one of the group consisting of copolymers ofhexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE).

According to another preferred embodiment, the propellant comprises60-68% potassium perchlorate.

According to another preferred embodiment, the binder comprises 1-3%ethyl cellulose and 0.6-1.4% terpolymers of the at least one of thegroup consisting of copolymers of hexafluoropropylene (HFP), vinylidenefluoride (VF2), and tetrafluoroethylene (TFE).

According to another preferred embodiment, the propellant consists of30-34% 5-aminotetrazole, 0.6-1.4% aluminum powder, 60-68% potassiumperchlorate, 1-3% ethyl cellulose and 0.6-1.4% terpolymers of the atleast one of the group consisting of copolymers of hexafluoropropylene(HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE).

According to another preferred embodiment of the propellantmanufacturing method according to the invention, after granulating anddrying the damp cake, the resulting intermediate product is furtherprocessed by densifying or tableting the dried propellant granules toform dry propellant tablets and re-granulating said dry propellanttablets to generate re-granulated dry propellant granules and thenpassing the re-granulated dry propellant granules through a sieve inorder to filter for re-granulated dry propellant granules of a desiredgranule diameter. This process allows modifying the sieve cut andtherefore allows tailoring of the ballistic properties of the propellantto meet customer requirements without requiring a change of theformulation. Depending on the sieve cut and therefore the size of thedensified granules the propellant generates gas faster or slower.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings show:

FIG. 1 the ballistic properties of a propellant according to theinvention in a diagram showing the pressure in psi after firing over thetime in milliseconds for a variety of propellants exposed to differenthigh-temperature 120° C. exposure time spans;

FIG. 2 the ballistic properties of a propellant according to the priorart in a diagram showing the pressure in psi after firing over the timein milliseconds for a variety of propellants exposed to differenthigh-temperature 120° C. exposure time spans; and

FIG. 3 an example of a typical micro-gas generator as known in the priorart for which the propellant according to FIG. 1 may be used.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 3 shows an example of a typical micro-gas generator as known in theprior art for which the propellant according to FIGS. 1 and 2 may beused. An initiator 3, including contact pins or lead wires 1, andcaptured within a molded body 2, is capable of conducting electriccurrent from an external source such as a control circuit that respondsto rapid deceleration, when the micro gas generator is used in anautomobile seat belt pre-tensioner to a metallic bridge wire or similar.When electrically energized with an appropriate signal, the initiator 3produces a high temperature arc or spark to initiate the explosion of aninitiation charge 4 surrounding a bridge wire. The molded body 2, formedwithin a retainer 5, is fastened to a propellant can 6 containing theoutput propellant 7. The gas pressure will finally rupture thepropellant can 6 when enough pressure has built up in the output can andrelease the gas to its intended destination, for instance an enginecompartment actuator assembly for lifting the hood in case of pedestrianimpact or in case of a seatbelt pretension to a rack and pinion deviceor a piston and steel cable device tightening the seatbelt.

In FIG. 2 the ballistic properties of a prior art propellant are shownin a diagram providing the pressure in psi after firing over the time inmilliseconds for a propellant for a variety of high-temperature 120° C.exposure times, namely i) no exposure, denoted by reference numeral 10,ii) exposure for 48 hours denoted by reference numeral 11; and iii)exposure over 1000 hours denoted by reference numeral 12. For thepurpose of this diagram, an experimental setting was used capturing andsensing the pressure of the firing. As FIG. 2 reveals, the graphs 11 and12 are close together, i.e. have a ballistic performance that is similarand distinguishes for both graphs 11 and 12 significantly from the graph10 demonstrating the ballistic properties of the propellant at noexposure to high temperature. The graphs 11 and 12 reveal that theeffect of significantly altering the ballistic performance of thepropellant is already for the most part completed after only 48 hours ofexposure to 120° C., i.e. an exposure beyond 48 hours, for instance 1000hours as shown in the graph 12, does not make any significant furtherdifference. As these graphs reflect, the 120° C. exposed propellantreaches the maximum pressure at point 13 at about 3 ms while thenon-temperature-exposed propellant reaches the maximum pressure at about8 ms at point 14. While the maximum pressures both for thetemperature-exposed propellants at point 13 and of thenon-temperature-exposed at point 14 are about the same, the negligibledifference may be explainable for the most part by dynamic effectsresulting from the very fast gas generation for the temperature-agedpropellant. As a conclusion, the prior art non-temperature-exposedpropellant builds up pressure more gradually, reaching peak pressureover a period of approximately 8 ms, as compared to thetemperature-exposed propellant of the same formulation which reachesthis same peak pressure in a period of only about 2 ms. The formulationof the prior art propellant shown in FIG. 2 contains 19%±1%5-aminotetrazole, 17%±1% azodicarbonamide 1%±0.2% aluminum powder,60%±1% potassium perchlorate, and 3%±0.5% ethyl cellulose.

FIG. 1 shows a diagram similar to FIG. 2 but in contrast to FIG. 2demonstrates the ballistic performance of a propellant according to theincident invention in a diagram providing the pressure in psi afterfiring over the time in milliseconds for the propellant for a variety ofhigh-temperature 120° C. exposure times, namely i) no exposure, denotedby reference numeral 16, ii) exposure for 48 hours denoted by referencenumeral 17; and iii) exposure over 2000 hours denoted by referencenumeral 18. As the diagram of FIG. 1 reveals, the maximum pressure isreached for all levels of temperature exposure at about the same pointdenoted 19 at about 8 ms like for the non-temperature exposure agedprior art propellant demonstrated by graph 10 in FIG. 2. This makes theapplication of the propellant according to the incident invention moreversatile, namely temperature exposure does not have any influence onthe ballistic performance of the propellant according to the incidentinvention that would go beyond negligible variations. Put in otherwords, the propellant according to the incident invention generates gasmore gradually in comparison with temperature-exposure aged propellantaccording to the prior art. The specific example demonstrated in FIG. 1has a formulation consisting of 32%±1% 5-aminotetrazole, 1%±0.2%aluminum powder, 64%±1% potassium perchlorate, 2%±0.5% ethyl celluloseand 1%±0.2% terpolymers of at least one of the group consisting ofcopolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE). The terpolymers of at least one of the groupconsisting of copolymers of hexafluoropropylene (HFP), vinylidenefluoride (VF2), and tetrafluoroethylene (TFE) are commercially availableas “Viton® B”, a composition manufactured and distributed by the DuPontPerformance Elastomers L.L.C. it is noted that the propellant accordingto the incident invention is not limited to these exact aforementionedranges of the propellant used in FIG. 1, meaning that a propellant with,to some extent altered ranges, provides a similar ballistic performanceirrespective of temperature-exposure aging.

The propellant with the ballistic performance shown in FIG. 1 wasmanufactured by adding acetone as a solvent to solvate ethyl celluloseand Viton® B=terpolymers of at least one of the group consisting ofcopolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE); evaporating the acetone using a low level ofvacuum until the mixture is a damp cake; and granulating and drying thedamp cake forming dried propellant granules. The dried propellantgranules were then densified/tableted to form dry propellant tablets andthe dry propellant tablets were then re-granulated to generatere-granulated dry propellant granules that were then passed through asieve in order to filter for re-granulated dry propellant granules of adesired granule diameter. In the alternative to densified granules, anumber of different applications are possible like pressed pellets,powder, extrusion, cast grains etc.

A wide variation of applications is possible for the propellantaccording to the incident invention. In context with a vehicle,specifically installing the micro-gas generator in the enginecompartment or close to other heat sources in a vehicle like thetransmission is possible, but also in connection with other safetydevice applications such as seat belt retractors, buckle pre-tensioners,airbag inflators, head rest actuators, seat interlocks, hood lifters,and other pedestrian protection devices that require high reliabilitygas generation devices. Other applications are envisaged such aspropellants for use in automotive inflator systems in a pressed orextruded tablet or grain form. Also applications in aerospace anddefense are envisaged such as thrusters, actuators, canopies and seatejection motor applications.

What is claimed is:
 1. Propellant for a micro-gas generator, thepropellant comprising 5-aminotetrazole, aluminum, a binder and anoxidizer and is substantially free from azodicarbonamide.
 2. Propellantaccording to claim 1, wherein the oxidizer comprises potassiumperchlorate.
 3. Propellant according to claim 1, wherein the bindercomprises ethyl cellulose as a binder constituent.
 4. Propellantaccording to claim 3, wherein the binder comprises fluoropolymers as abinder constituent.
 5. Propellant according to claim 4, wherein thefluoropolymers are terpolymers of at least one of a group consisting ofcopolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE).
 6. Propellant according to claim 1,comprising 30-34% 5-aminotetrazole.
 7. Propellant according to claim 1,comprising 0.6-1.4% aluminum powder.
 8. Propellant according to claim 1,the binder comprising 0.6-1.4% terpolymers of the at least one of thegroup consisting of copolymers of hexafluoropropylene (HFP), vinylidenefluoride (VF2), and tetrafluoroethylene (TFE).
 9. Propellant accordingto claim 2, comprising 60-68% potassium perchlorate.
 10. Propellantaccording to claim 5, the binder comprising 1-3% ethyl cellulose and0.6-1.4% terpolymers of the at least one of the group consisting ofcopolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE).
 11. Propellant according to claim 1,consisting of 30-34% 5-aminotetrazole, 0.6-1.4% aluminum powder, 60-68%potassium perchlorate, 1-3% ethyl cellulose and 0.6-1.4% terpolymers ofthe at least one of the group consisting of copolymers ofhexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE).
 12. A method of manufacturing a propellantfor a micro-gas generator, comprising: providing a propellant mixture of5-aminotetrazole, aluminum powder, potassium perchlorate, ethylcellulose and fluoropolymers; adding acetone as a solvent to solvateethyl cellulose and fluoropolymers; evaporating the acetone using a lowlevel of vacuum until the mixture is a damp cake; and granulating anddrying the damp cake forming dried propellant granules.
 13. The methodaccording to claim 12, wherein the step of granulating and drying thedamp cake is the last step for making the final propellant.
 14. Themethod according to claim 12, further comprising densifying or tabletingthe dried propellant granules to form dry propellant tablets andre-granulating said dry propellant tablets to generate re-granulated drypropellant granules; and passing the re-granulated dry propellantgranules through a sieve in order to filter for re-granulated drypropellant granules of a desired granule diameter.
 15. The methodaccording to claim 12, further comprising composing the propellantmixture such that it consists of 30-34% 5-aminotetrazole, 0.6-1.4%aluminum powder, 60-68% potassium perchlorate, 1-3% ethyl cellulose and0.6-1.4% terpolymers of at least one of the group consisting ofcopolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), andtetrafluoroethylene (TFE).
 16. The method according to claim 12, furthercomprising composing the propellant mixture such that it comprises30-34% 5-aminotetrazole, 0.6-1.4% aluminum powder, 60-68% potassiumperchlorate, 1-3% ethyl cellulose and 0.6-1.4% terpolymers of at leastone of the group consisting of copolymers of hexafluoropropylene (HFP),vinylidene fluoride (VF2), and tetrafluoroethylene (TFE).